Well, I had to rule out the stupid!
Ooh, this is a good riddle. Right, so <em>most</em> people can't tell what is in the box by sight. Whatever is in the box is silent and undetectable by smell. It is a small, regular rectangular, box so I guess touch isn't going to help. Surely not taste?
So I'm stumped!!
Do you know what is in the box by sight alone?